Used front center ones are easy to find but in all the years I've been selling parts I've only sold one. I think I got $30 for it.

If you have a good used rear center filler you can pretty much set your own price because you'll have 1 of only a handful left.

The lower corner fillers are simple to find too and don't sell for much.

Replica Plastics makes the fiberglass parts that Classic Industries sells. All those pieces have been available for years. Before CI and a few other places started selling them, RP sold them direct to the public. They no longer do so now.

I got a rear center filler from them back in 07(?). See this thread - https://www.gmbbodyforum.com/t8-bumper-fillerAt some point they re-designed it. It's supposed to resemble the original one better now.